For several years it’s been known that there were various crossbreeding episodes between non-African homo sapiens, the modern human being, and Homo neanderthalensis, the one commonly called Neanderthals. Today, between 2% and 4% of the DNA of non-African modern humans is inherited from Neanderthals. It’s a small amount and this research had the purpose of trying to assess the force of natural selection against other Neanderthal genes.

In biology, when there’s a crossbreeding event between two different species the phenomenon of introgression occurs. In essence, the hybrids crossbreed again with at least one of the two parental species forming other hybrids. With the passing of generations, these crossbreedings introduce in the species various gene combinations of the other species. In the case of the Sapiens-Neanderthal hybrids, natural selection worked against introgression.

A research published in the journal “Genetics” in June 2016 already examined the phenomenon of introgression in the Sapiens-Neanderthal hybrids arguing that Neanderthals had average health levels lower than homo sapiens with consequences at genetic level. Ivan Juric team further investigated this issue to try to improve our knowledge on the contributions of Neanderthal to modern humans’ DNA.

This was a complex investigation because the different populations of modern humans outside Africa have slightly different amounts of DNA inherited from Neanderthals. For example, one conclusion is the confirmation that East Asian populations had a greater amount of Neanderthal ancestors than Europeans.

The most important conclusion of this research concerns the differences between the population of homo sapiens and Neanderthals. Even many thousands of years ago, when the crossbreeding between the two species occurred, there were many more modern humans than Neanderthals. It’s a crucial factor because natural selection works better on a large population.

Basically, among Neanderthals various weakly deleterious genes kept on remaining in their DNA because their population was small. The same genes were eliminated after being inherited by Homo sapiens because their population was larger so it was more likely that individuals with better genes survived.

According to the researchers, the conclusions of research and in particular the demographic differences between modern humans and Neanderthals don’t explain everything. An example are the possible genes that could be deleterious for hybrids only. Genetic research on hybrid between modern humans and other hominids now extinct are still at the beginning so in the coming years we can expect more developments.